The present invention relates to the field of running shoes, and, in particular, to a sole construction for running shoes designed to reduce problems of excess "pronation" and heel wear.
As described in detail in my book entitled The Running Shoe Book, Anderson World, Inc., 1980, during running, initial contact between a runner's shoe and the ground occurs at the outside or lateral edge of the shoe and not the back edge thereof, as occurs during walking (see upper illustration, FIG. 4). After landing on the lateral border of the shoe, the foot and the shoe tend to shift quickly into a flat position (central illustration, FIG. 4). This flattening-out of the foot involves the subtalar joint (which is the joint between the talus and the heel bone). From the flat position, this side-to-side rolling motion then continues into a condition known as "pronation" (lower illustration, FIG. 4), wherein the foot is angled inwardly upon its inside edge. This side-to-side rolling movement into pronation causes trouble only when pronation does not stop within what is considered a normal range. Because pronation involves the rotation of the subtalar joint, it involves both the leg and the foot, with the result that the subtalar joint pronates, as the leg rotates inward. If there is too much pronation, a large amount of inward rotation will occur, and this will produce a screwing type of motion at the knee joint. Since the knee is not designed to resist this type of screwing motion, when excessive amounts of pronation occur, the runner's knee joint is likely to be injured.
To prevent side-to-side rolling of the foot during the support phase (flat position), lateral flaring of the sole of running shoes was introduced in 1975. Typical lateral flaring is shown in FIGS. 4 and 6, wherein it can be seen that the midsole starts out wide at the base and gradually tapers up toward the featherline where the sole is joined to the upper of the shoe. In my above-noted book, I pointed out that there was no reason that the flare should be symmetrical on the inside and outside border of the shoe, and that the outer flare could be reduced, because the most resistance is required on the inside to prevent inward rolling motion. Implementation of this idea can be seen in the FIG. 4 embodiment of U.S. Pat. No. 4,255,877, wherein the midsole is flared on the inner side of the heel, but the outer edge of the sole is squared.
Additionally, since the outside border of the shoe, and not the back edge, is the first in line to receive pressure at initial contact, the runner wearing a running shoe having a flared or squared outside edge is landing on a soft "knife edge" of the lateral border of the shoe, and a smooth transition between the contact and support phases is not likely to be achieved, particularly if the heel strike occurs somewhat outwardly of the actual edge. In this regard, this particular problem can, perhaps, be best visualized if the rolling motion is equated to that which occurs if a square cylinder or wheel is rolled about its longitudinal axis.
Furthermore, because of the above-noted location of initial heel contact with the ground, the most common location for outsole wear is the rear outside border of the shoe. This wear occurs, principally, when the foot first contacts the ground and relative movement occurs between the shoe and the ground, which produces a grinding effect that wears away the outside corner of the heel in the area indicated by the broken line in FIG. 5.
U.S. Pat. No. 4,241,523 shows a shoe that has an outer sole with a convex curvature for facilitating a rolling motion of the foot and the shoe after initial contact of the foot of the wearer with the ground. However, since this rocker provides a rolling about a laterally-extending axis at the rear of the heel, and since this rolling motion is produced by a rocker that is described as necessarily being relatively rigid and having only a limited amount of flexibility, the sole construction of this patent is unsuitable for running shoes. That is, due to the fact that heel strike occurs at the lateral outer edge of the heel and produces a side-to-side rolling of the foot (as opposed to the back-to-front rolling which occurs after heel strike during walking), the rocker utilized in accordance with this patent is ineffective with regard to this typical running motion, while the rigid construction of the rocker will inhibit satisfactory cushioning of the foot during heel strike.